Clostridium difficile (C. difficile) infection (CDI) is a serious disease caused by colon infection with C. difficile bacteria in humans. There are ˜500,000 CDI infections per year in the U.S., resulting in 15,000-20,000 deaths. The healthcare cost for CDI is estimated to be $4.8 billion (See Rupnik et al., Nat. Rev. Microbiol., 2009, 7, 526-36; Dubberke et al., Clin. Infect. Dis., 2012, 55, S88-92). Recurrent CDI affects about 30% of patients (Rupnik et al., N. Engl. Med., 2011, 364, 422-31). CDI is primarily caused by: (i) the disruption of the gut microbiome by the broad-spectrum antibiotic therapy; and (ii) the failure to produce neutralizing anti-toxin antibodies in patients (Chang et al., J. Infect. Dis., 2008, 197, 435-38; Kyne et al., Lancet, 2001, 357, 189-93). C. difficile bacteria produces two main toxins (namely, toxin A and toxin B), which glucosylate Rho-GTPases inside the epithelial cells, and lead to fever, abdominal pain, diarrhea, and colon inflammation (Rupnik et al., Nat. Rev. Microbiol., 2009, 7, 526-36).
Treatment of CDI focuses on the use of several different antibiotics. These antibiotics include: (i) vancomycin (FDA-approved in 1986); (ii) metronidazole (not FDA-approved but is used “off-label”); and (iii) fidaxomicin (FDA-approved in 2011), a more narrow-spectrum antibiotic in comparison to vancomycin or metronidazole. Fidaxomicin reduces CDI recurrence from 25% (with vancomycin treatment) to 8% (with fidaxomicin treatment) (Louie et al., N. Engl. J. Med., 2011, 364, 422-31). Recently, there are reports indicating the rise of new strains of C. difficile bacteria that exhibit an increased toxin production and resistance to fluoroquinolones (Lessa et al., Clin. Infect. Dis., 2012, 55, S65-70). For example, the epidemic BI/NAP1/O27 strain now accounts for 51% of isolates from hospital outbreaks in the U.S. (McDonald et al., N. Engl. J. Med., 2005, 353, 2433-41). There is a need for developing new narrow-spectrum antibiotics in treating CDI. Several narrow-spectrum antibiotics are in pre-clinical and early clinical development; however, their efficacy remains unknown.
An alternative means to treat CDI involves the use of anti-toxin antibodies. Active immunization using bacterial toxin as a vaccine is presently in clinical trials. However, the efficacy remains unknown. Passive immunization with monoclonal anti-toxin antibodies has been reported to reduce CDI recurrence from 32% to 8% when used in combination with vancomycin in a Phase II clinical trial (Lowy et al., N. Engl. J. Med., 2010, 362, 197-205). The high cost of monoclonal antibody may hinder the immunization approach for its clinical application in non-hospitalized CDI patients, which account for 42% of cases in a large study (8,569 patients) (Kasper et al., Infect. Control Hosp. Epidemiol., 2012, 33, 470-76).
Another emerging approach involves the use of novel compounds that inhibit the C. difficile toxins directly. Pruitt et al. disclose inositol hexaphosphate (InsP6) can induce an autocatalytic cleavage of C. difficile toxin A (J. Biol. Chem., 2009, 284, 21934-40). Savidge et al. showed that S-nitrosylation of the toxins inhibits the toxin activity in vitro (Nat. Med., 2011, 17, 1136-41). These studies open up windows towards identifying novel compounds that can neutralize the toxin's activity. Abdeen et al. disclose a peptide that is active against both C. difficile toxin A and toxin B (ACS Chem. Biol., 2010, 5, 1097-1103). Puri et al. disclose the rational design of covalent small molecule inhibitors of C. difficile toxin B (Chem. & Biol., 2010, 17, 1201-11). In particular, di-peptides and tri-peptides have been identified to uniquely inhibit the cysteine protease domain of toxin and reduce the toxicity. However, there has been no report regarding the in vivo activity of these compounds. An oral toxin-binding polymer (Tolevamer) has been developed to bind to C. difficile toxins, but was reported to fail in a Phase II trial (Johnson et al., Antimicrob. Agents Chemother., 2012, 56, 4043-45).
Accordingly, there is a continuing need for identifying a novel compound that can inhibit C. difficile toxins and thus provide a new approach for CDI treatment.